Chireix and Doherty amplifiers were the first examples of RF amplifiers based on multiple transistors with passive output network interaction and combination that gave high average efficiency for amplitude-modulated signals.
FIG. 1A shows a Chireix amplifier 10, which is called an “outphasing” system. The Chireix amplifier 10 uses a signal separator 12 to divide an input signal Sin(t) including amplitude and phase modulation into two constant envelope phase-modulated branch signals S1(t) and S2(t). The divided branch signals are amplified by respective similar power amplifiers (PA) 16A and 16B, and a power combiner 18 sums the branch signals to produce an amplified version of the input signal having a magnitude based on the phase difference between them. This combined signal is provided to an output load RL, such as an antenna or other load device. For example, if Sin(t)=A(t), then S1(t)=cos [ωt+cos−1 [A(t)]}, S2(t)=cos [ωt−cos−1 [A(t)]}, and S1(t)+S2(t)=2A(t)cos(ωt). If the PAs 16A and 16B have the same gain G, the output signal would be S1(t)+S2(t)=2GA(t)cos(ωt). Because PAs 16A and 16B operate at a fixed power level (fixed amplitude), they can be highly non-linear.
FIG. 1B depicts a two-stage Doherty amplifier, which combines dissimilar, but linear PAs called a “main” (carrier) PA 26A and an auxiliary (peaking) PA 26B. The outputs of the PAs 26A and 26B are combined through a transmission line coupler 28 A. A quarter-wave line 28B on the input of the auxiliary PA 26B compensates for the quarter-cycle phase shift in the transmission line on the output of main PA 26A. At low output levels, the drive to the auxiliary PA 26B is cut off, typically at 6 dB from the maximum composite power, and the main PA 26 A operates as a linear class-B amplifier. The impedance presented to the main amplifier 26A by the coupler 28A saturates the main amplifier 26A at a point well below the system peak output power. This results in maximum amplifier efficiency at the transition point and the system peak output power, and high average efficiency for full-carrier AM signals.
Multiple transistor amplifiers based on passive output network interaction structures have the general advantage of needing only fundamental (RF) frequency network and signal modifications. Compared with single-transistor amplifiers, they differ only in the number of independently driven transistors. Harmonics and/or baseband modifications, which are required for other high-efficiency amplifiers, are optional.
The field was generalized for two-transistor structures in U.S. Patent Application Publication Nos. 2003137346 and 2004051583 and three basic expandable multi-transistor structures (and ways to drive them efficiently) International patent application WO2004/023647, International patent application WO2004/057755, and International patent application WO2004SE01357 have been patented. These structures are sufficient to provide all combinations of Doherty and Chireix features. (For a detailed description of background and developments refer to the referenced documents.)
Existing solutions to the problem of combining Chireix amplifiers calls for either connecting several Chireix pairs to the same output or putting them in a Doherty structure. For optimal utilization of the transistors, this requires certain specific relations between the sizes of the transistors. These relations may be hard to combine with available transistor sizes without output power overhead.
Another consideration is that, although the previously invented Chireix/Chireix and Chireix/Doherty combination structures are sufficient for building amplifiers with very high efficiency for all conceivable multi-carrier and multi-user signals, the new structures disclosed herein present another way to increase amplified signal efficiency.
Another problem is that of transistor shunt loss. This loss is due to resistive parasitics that couple from the output node (drain, collector) of the devices to ground (source, emitter), and is proportional to the node voltage squared. Thus, the problem of shunt loss is exacerbated by the operation of the multi-transistor amplifiers, which decrease the transistors' RF output currents (which is the reason for their high efficiency) at the expense of increased RF output voltages. High RF voltages give high loss in the shunt parasitic resistance, and this loss power is higher relative to the output power at low and medium outputs. In other words, the shunt loss degrades efficiency more at low outputs for these types of multi-transistor amplifiers.